Just following up in case it helps any future readers. This problem is solved. The issue was that I was dragging the VST from "effects" rather than "generators".

I really like the sound of the heavier7strings vst. I installed the demo, and it's causing me some weird issue I've never seen before. I'd be eternally grateful to anyone who can tell me how to fix this! (FL Studio v20.9.2, Mac version) I drag "Heavier7StringsVST" from the plugin database ...